This invention relates to smart pipeline inspection gauges, commonly termed xe2x80x9csmart pigs,xe2x80x9d used in the inspection of pipelines.
Pigs are devices that are moved through a pipeline by the fluid pressure within the pipeline to provide information regarding the condition of the pipeline. This can vary between simple tasks, such as cleaning pipelines to more sophisticated determinations such as measurement of metal loss of the pipe due to corrosion, cracks, deformation and the like. Pigs that perform these tasks are called xe2x80x9csmart pigsxe2x80x9d. Smart pigs may consist of various modules, in which one of the modules performs the function of propelling the smart pig through the pipeline. With respect to determining metal loss in the pipe, the industry standard is to use the technique of Magnetic Flux Leakage (MFL). With this technique, the speed of the pig cannot exceed 7 mph or otherwise the quality of the MFL measurement is degraded. For this purpose, it is customary to reduce the volumetric throughput of the pipeline to obtain the proper pig speed and thus achieve the desired high quality of inspection. This is undesirable because it also results in reduced production. For example, in the case of gas pipelines, the volumetric throughput can typically reach speeds up to 25 mph. To reduce the adverse affect on production and to maintain integrity of the MFL measurements, it is necessary to otherwise control the speed of the pig passing through the pipeline and maintain production through the pipeline. In gas pipelines it is known to do this by varying the gas bypassing through the pig. Conventional devices for performing this function are shown in U.S. Pat. No. 5,208,936, issued May 11, 1993. Although prior art mechanisms, such as the one disclosed in the aforementioned patent, are used for this purpose, their use is not practical at the high gas flow rates encountered in gas pipelines, because they exhibit a narrow controllable pressure drop range that limits the product flow conditions with which these mechanisms may be effectively used.
It is accordingly an object of the present invention to provide a more efficient mechanism for allowing flow to bypass through the smart pig at high velocities occurring in present day gas pipelines, while effectively and accurately controlling the speed of the pig at the lower limits required for high quality MFL inspection. This is achieved by the use of a plurality of venturi-shaped through passages for controlling the flow of fluid through the pig. It has been determined that by the use of venturi-shaped passages for this purpose turbulence, loss of fluid energy, and momentum are avoided and results in recovery of static pressure which does not occur with prior art devices. When in the full-open position the venturi-shaped passages provide maximum reduction in flow loss. By providing a more efficient mechanism for this purpose, the allowable flow range of the pig may be increased. This efficiency is necessary when the mechanism is in the maximum bypass position to operate the pig at low speeds relative to the gas flow rate through the pipeline.
The bypass of fluid, including gas, through the pig creates a pressure drop or pressure differential. This pressure differential, as is well known, propels the pig through the pipeline. Additional factors that affect the movement of the pig through the pipeline are friction and elevation. Thus, using Newton""s Laws of Motion, the velocity and acceleration of the pig is governed by the following equations:
Mxc2x7a=xe2x88x92Ffriction+Fpressurexe2x80x94dropxc2x1FelevationV=Vo+at
where
M=Mass of smart pig
a=acceleration of smart pig
Ffriction=Frictional force as a result of smart pig-to-pipeline interaction
Fpressurexe2x80x94drop=Force acting on smart pig as a result of fluid passing over and through the
smart pig
Felevation=Gravitational force acting on the smart pig in reference to a predetermined
neutral plane.
V=velocity of smart pig
Vo=previous velocity state of smart pig
t=elapsed time between previous and present states
From these equations, it may be seen that the velocity of the pig is determined by the frictional force, pressure drop and inclination/elevation of the pipeline. To permit the pig to operate at the low speeds necessary for effective MFL measurements, which is below 7 mph, the parameter easiest to control is the pressure drop across the pig. This is achieved by bypassing the majority of the gas through the pig, which in turn requires minimizing the pressure drop through the pig. In accordance with the invention, this is achieved by the use of a plurality of venturi-shaped passages through which fluid passing through the pig is introduced. This has been found to provide an accurate and simple mechanism for controlling pressure drop, particularly when the fluid is gas.
Specifically with methane gas at 714.5 psi operating pressure and a temperature of 25 C the maximum gas speed would be 11 mph with the maximum speed of the pig being at 7 mph, with conventional structures. Under these identical conditions, using a venturi-shaped passage in accordance with the invention, gas speeds to 20 mph maybe encountered while maintaining the pig speed at 7 mph maximum.
In accordance with the invention there is provided a variable speed pig for movement within a pipeline that has a cylindrical housing with an annular seal circumferentially mounted to the housing for sealing engagement between it and the pipeline. A plurality of venturi-shaped through passages extend longitudinally within the housing to receive flow passing through said pig. Means are provided for varying the size and shape of the passages to vary the pressure drop through the passages and pig to correspondingly vary the speed of the pig through the pipeline.
Each of the passages may have a tapered portion to recover a portion of pressure loss after said pressure drop through said through passage.
The passages each have a plurality of restrictions shaped to define a venturi opening within each of the passages.
In one embodiment of the invention, the through passages are disposed within the housing in spaced-apart circumferential relationship.
One embodiment for varying the size and shape of the passages includes a rotatable component.
Another embodiment for varying the size and shape of the passages, includes a component having selectively restricted portions and open portions for selective engagement with the passages to block portions of these passages to vary the size thereof.
The component and the passages may be mounted for relative movement.
The means for providing relative movement of the component and passages may be contained within the housing of the pig.
An embodiment of the invention provides that the component and the passages are axially mounted for relative movement.
In another embodiment of the invention, the size and shape of the openings through the passages may be varied by the use of a plurality of axially movable components. These axially movable components may be used with a plurality of fixed components, with the axially movable components being mounted for axial movement relative to the plurality of fixed components.
In yet another embodiment of the invention for varying the size and shape of the openings through the passages, a plurality of spaced-apart fixed components may be used that contain therein a component for selectively increasing and decreasing a portion of the fixed components for selective engagement and disengagement to vary the size of the openings through the passages. The component contained within the fixed components may be a rotatable interior component mounted within the fixed components for rotation between an axial position relative to the longitudinal axis of the fixed components and a position normal to this axis at which in this later position the rotatable interior component increases a portion of the fixed component.
Various supplemental means may be provided for varying friction between the pig and the pipeline to additionally vary the speed of the pig through the pipeline.